Water-soluble dyes are commonly used for ink-jet printers in the form of a composition comprising the dye, water, and a water-miscible organic solvent, such as a glycol ether. A typical composition comprises water and ethylene glycol in equal proportions and up to about eight percent of a dye such as Food Black 2. The amount of dye is determined by the density of the print required. Such compositions are formulated to have, among other things, the viscosity and surface tension required for the ink-jet printing process. In addition, an important feature of such a composition is that the dye must have good water solubility, not only to provide the required print density, but also to avoid precipitation of the dye in the ink-jet nozzles to form a crust which blocks the nozzle. The latter problem arises when some of the water in the ink composition contained in the nozzle evaporates and the dye precipitates from solution to leave a crust in the nozzle. Over a period of time, this crust builds and causes partial and subsequently full blockage of the nozzle.
One approach to solving this problem has been to incorporate sulfonate groups, SO.sub.3, in the dye with the associated cation, such as sodium, potassium, lithium, ammonium, and amine salt cations. Such dyes are described, for example, in U.S. Pat. No. 4,557,761. However, dyes with good water solubility generally have three disadvantages with regard to their interaction with paper. First, such dyes are easily smeared on contact by fingers or any moisture. Second, they interact poorly with paper, which causes them to migrate into the paper with the dye's solvents to give poor, diffuse print quality. Finally, the failure of these dyes to interact with and bind to the paper causes inks formed from these dyes to have longer dry times.
Another approach to this problem of the water solubility of dyes has been to attach amino groups to the carbon skeletal structure of the dye molecule to form aromatic amines. Such aromatic amino groups protonate to form a charged species, thus imparting water solubility to the dye. However, such dyes are soluble only at pH levels below 2, which is too acidic for ink since it would adversely affect the paper and printer. Similar dyes possibly having methylene amino groups (--CH.sub.2 --NH.sub.2) attached to the aromatic nucleus are soluble at pH levels below 5 and provide good print quality and waterfastness. However, inks with a pH below 5 are still considered too acid for ink-jet printing systems, where a neutral pH is ideal in order to avoid corrosion and to be compatible with materials used in the ink-jet printer.
Thus, a need exists in the industry for a dye which is soluble at a pH that is suitable for use in ink-jet printing, which has good water solubility, while at the same time is able to interact strongly with paper.